1. Field of the Invention
The present invention relates to a multiple heat pump, and more particularly, to a control method for a multiple heat pump which can return a liquid refrigerant, remaining in shutdown indoor units, into compressors when only one of multiple indoor units operates in a heating mode.
2. Description of the Related Art
FIG. 1 is a schematic cycle diagram illustrating refrigerant flow in a cooling mode of a conventional multiple heat pump. FIG. 2 is a schematic cycle diagram illustrating refrigerant flow in a heating mode of the conventional multiple heat pump. FIG. 3 is a schematic cycle diagram illustrating refrigerant flow when one of multiple indoor units of the conventional multiple heat pump operates in a heating mode and the other indoor units shut down.
As shown in FIGS. 1 to 3, the conventional multiple heat pump comprises multiple indoor units 1, 2, 3 and 4. Each of the indoor units 1, 2, 3 or 4 is provided with an indoor blower 5, 6, 7 or 8 that suctions indoor air thereinto and discharges it again to a room, and an indoor heat exchanger 11, 12, 13 or 14 that heat exchanges the indoor air, suctioned into the indoor unit, with a refrigerant so as to heat or cool the air.
The conventional multiple heat pump further comprises a single outdoor unit 20 including compressors 22, an accumulator 26, oil separators 30, an outdoor heat exchanger 34 and a four-way valve 38. The compressors 22 are used to compress a refrigerant, and the accumulator 26, connected to refrigerant suction pipes 24 of the compressors 22, is used to accumulate a liquid refrigerant in order to permit only a gas refrigerant to be introduced into the compressors 22. The oil separators 30 are connected to refrigerant discharge pipes 28 of the respective compressors 22 in order to separate oil discharged together with the refrigerant from the compressors 22. The outdoor heat exchanger 34 is used to heat exchange the refrigerant with outside air. The four-way valve 38 is connected to the oil separators 30, indoor heat exchangers 11, 12, 13 and 14, accumulator 26 and outdoor heat exchanger 34 via refrigerant pipes 36a, 36b, 36c and 36d, and is used to switch a refrigerant channel in order to selectively send the refrigerant, passed through the oil separators 30, to the indoor heat exchangers 11, 12, 13 and 14 or outdoor heat exchanger 34.
An additional refrigerant pipe 36 is provided to directly connect the outdoor heat exchanger 34 to the respective indoor heat exchangers 11, 12, 13 and 14. The refrigerant pipe 36 is provided with an expansion mechanism that expands the refrigerant, passed through the outdoor heat exchanger 34 or indoor heat exchangers 11, 12, 13 and 14, to a low-temperature and low-pressure refrigerant.
The expansion mechanism includes indoor electronic expansion valves 15, 16, 17 and 18 mounted in the respective indoor units 1, 2, 3 and 4 to permit the refrigerant passing therethrough to expand in cooling/heating modes, and an outdoor expansion device 40 mounted in the outdoor unit 20 to permit passage of the refrigerant only in the heating mode.
The outdoor expansion device 40 includes a check valve 42, a bypass pipe 44, and an outdoor electronic expansion valves 46. The check valve 42 is provided at the refrigerant pipe 36 connected to the outdoor heat exchanger 34 and is used to pass the refrigerant in the cooling mode and obstruct the refrigerant in the heating mode. The bypass pipe 44 serves to divert the refrigerant obstructed by the check valve 42, and the outdoor electronic expansion valve 46 serves to expand the refrigerant passing through the bypass pipe 44.
Now, the operation of the conventional multiple heat pump configured as stated above will be explained.
When all of the indoor units 1, 2, 3 and 4 operate in a cooling mode, as shown in FIG. 1, the four-way valve 38 is switched to send a high-temperature and high-pressure gas refrigerant, emerged from the compressors 22, to the outdoor heat exchanger 34. While passing through the outdoor heat exchanger 34, the high-temperature and high-pressure gas refrigerant is heat exchanged with the surrounding air, thereby being condensed to a liquid refrigerant. The liquid refrigerant is transferred to the respective indoor units 1, 2, 3 and 4 through the check valve 42.
The liquid refrigerant, transferred to the respective indoor units 1, 2, 3 and 4, is expanded to a two-phase refrigerant containing both liquid and gas by the indoor electronic expansion valves 15, 16, 17 and 18, and then is introduced into the indoor heat exchangers 11, 12, 13 and 14 of the respective indoor units 1, 2, 3 and 4. While passing through the indoor heat exchangers 11, 12, 13 and 14, the two-phase refrigerant absorbs the surrounding heat as it is evaporated to a refrigerant vapor, thereby allowing the multiple indoor units 1, 2, 3 and 4 to function as coolers. Meanwhile, the refrigerant vapor, passed through the indoor heat exchangers 11, 12, 13 and 14, is transferred again to the outdoor unit 20, and is sent to the accumulator 26 by the four-way valve 38, thereby being finally circulated to the compressors 22. In this way, a cooling cycle is completed.
On the contrary, when all of the indoor units 1, 2, 3 and 4 operate in a heating mode, as shown in FIG. 2, the four-way valve 38 is switched to send a high-temperature and high-pressure gas refrigerant, emerged from the compressors 22, to the respective indoor units 1, 2, 3 and 4, opposite to the above described cooling mode.
The high-temperature and high-pressure gas refrigerant, transferred to the respective indoor units 1, 2, 3 and 4, emits heat to the surroundings as it is condensed to a liquid refrigerant while passing through the indoor heat exchangers 11, 12, 13 and 14, thereby allowing the multiple indoor units 1, 2, 3 and 4 to function as heaters.
The liquid refrigerant, passed through the indoor heat exchangers 11, 12, 13 and 14, is expanded to a two-phase refrigerant containing both liquid and gas by the respective indoor electronic expansion valves 15, 16, 17 and 18, and then is transferred to the outdoor unit 20.
The two-phase refrigerant, transferred into the outdoor unit 20, passes the bypass pipe 44 since it is obstructed by the check valve 42. Thereby, the refrigerant is expanded by the outdoor electronic expansion valve 46 provided at the bypass pipe 44, and is introduced into the outdoor heat exchanger 34, so that it is evaporated to a refrigerant vapor as it is heat exchanged with the surrounding air while passing through the outdoor heat exchanger 34. The refrigerant vapor is sent to the four-way valve 38.
The refrigerant vapor, sent to the four-way valve 38, is circulated to the compressors 22 after passing through the accumulator 26, completing a heating cycle.
Meanwhile, such a conventional multiple heat pump air conditioning system operates in such a fashion that one of the multiple indoor units 4 operates in a heating mode and the other indoor units 1, 2 and 3 shut down. In this case, the electronic expansion valve 18 of the indoor unit 4, operating in the heating mode, is controlled to attain a desired opening degree higher than a standard opening degree, whereas the electronic expansion valves 15, 16 and 17 of the shutdown indoor units 1, 2 and 3 are closed to the standard opening degree.
As can be seen from FIG. 3, the liquid refrigerant, passed through the indoor heat exchanger 14 of the operating indoor unit 4, is expanded to a low-temperature and low-pressure refrigerant while passing through the indoor electronic expansion valve 18 of the operating indoor unit 4, and then is circulated to the compressors 22 by successively passing through the outdoor electronic expansion valve 46, outdoor heat exchanger 34, four-way valve 38 and accumulator 26 of the outdoor unit 20. On the other hand, the electronic expansion valves 15, 16 and 17 of the shutdown indoor units 1, 2 and 3 are closed. This causes the liquid refrigerant to remain in the indoor heat exchangers 11, 12 and 13 of the shutdown indoor units 1, 2 and 3.
The fact that the liquid refrigerant remains in the indoor heat exchangers 11, 12 and 13 of the shutdown indoor units 1, 2 and 3 when only the indoor unit 4 operates in the heating mode means that a lesser amount of refrigerant is circulated to the compressors 22, causing a reduced cooling efficiency and overheating of the compressors 22. Such an overheating of the compressors 22 increases an outlet side temperature of the compressors, resulting in a deterioration of heating performance as well as damage and shorter life-span of the compressors 22.